Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a matrix or series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Systems that read and decode bar codes employing CCD or CMOS-based imaging systems are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging systems include CCD arrays, CMOS arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. Light reflected from a target image, e.g., a target bar code is focused through a lens of the imaging system onto the pixel array. Output signals from the pixels of the pixel array are digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and attempts to decode the imaged bar code.
The ability of an imaging system to successfully decode an imaged bar code is dependent upon the ability to satisfactorily capture a clear image of the target bar code that is focused onto the pixel array. Generally imaging scanners do not have movable lenses. They depend on using a small aperture such that mildly out-of-focus objects (barcodes) do not appear excessively blurred. A class of scanners or bar code readers sold by the assignee of the present invention use a bi-focal system, in which the lens is driven by an electro-magnetic motor, to one of two stable positions. U.S. Pat. No. 6,766,955 describes such a bi-focal system and is incorporated herein by reference.
While auto-focus in a bar code reader is a desirable feature, commercially available auto-focus sub-systems, such as those commonly used in digital cameras and camcorders, suffer from large size, slow action, and/or high power requirement.
Whether the imaging system is housed in a handheld, portable bar code reader or a permanently mounted reader, the user of the device cannot be expected to manually focus the imaging system by moving the lens, thus, there is a need for an automatic focusing system or auto focus system for an imaging system.
Bar code imaging systems require a variable focus optical system to maximize barcode reading range and deliver high quality images over a range of distances. The high scan rate for barcode reading imposes a high-speed requirement on the auto focusing technique to be used in the imaging system.
A typical two-dimensional barcode imaging scanner has an aiming pattern generator for the user to aim the scanner at the target and a separate illuminating system for illuminating the entire two-dimensional field of view. One auto-focusing technique that uses this aiming pattern is described in U.S. published patent application 2006:0038017 published on Feb. 23, 2006, which is assigned to the assignee of the present invention, and incorporated herein by reference in its entirety.
This above published application improves imaging quality through use of an auto-focus mechanism. Use of a moving lens tends to increase the device size. It is therefore desirable to create a compact auto-focus mechanism that does not appreciably increase device dimensions in relation to existing fixed focus imaging devices.
U.S. Pat. No. 6,905,068 is also assigned to the assignee of the present invention and is incorporated herein by reference. The '058 patent describes focusing an electro-optical reader for reading indicia by moving one prism relative to another prism. An outgoing laser beam directed to indicia has a return image focused by the moving prism.